The surface of lithographic aluminum plates are grained to provide proper adhesion to photosensitive layers and wettability to printing inks. In many of the present day graining processes at least one treatment involves the use of an electrolytic acid bath wherein an alternating current is passed between the aluminum plate and a suitable counter-electrode.
Such an electrolytic technique utilizes shaped alternating current wave forms in order to obtain improved graining. One such process is shown in U.S. Pat. No. 4,294,672 to Ohba et al., issued Oct. 13, 1981; the disclosure of which is incorporated herein by reference. This patent discloses a method wherein a zero voltage intermission is shown in the positive or negative phases of the alternating wave form. The quantity of energy is biased to the anodic phase. An aluminum plate so treated will exhibit a roughened surface having a double pit structure wherein pits will develop within pits. Such an irregular surface has been found to provide excellent adhesion to photosensitive coatings.